Knowledge Sharing | Fault Diagnosis and Handling of Automation Instruments

Instrument malfunctions typically manifest as abnormal indication values, such as excessively high or low readings, no change, or abnormal fluctuations. The causes of these faults fall into two categories:

  1. Abnormal process parameters due to unreasonable settings.
  2. Measurement system failures leading to inaccurate readings.

To ensure normal instrument operation, accurate fault diagnosis and analysis are essential. This requires:

  1. Knowledge of the measurement principles, instrument selection, and operational characteristics.
  2. Familiarity with the production process, material properties, and equipment/container characteristics.
  3. Understanding the workflow of the measurement system.

01. Fault Diagnosis Methods for Automation Instruments

Common diagnostic methods include:

  1. Visual Inspection: Check for physical damage to the instrument or cables, and ensure buttons/knobs on control panels function properly.
  2. Internal Inspection: Examine internal connectors, terminals, and pins for oxidation, corrosion, or poor contact. Verify the condition of fuses and relays.
  3. Voltage/Current Measurement: Use a multimeter to test voltage, current, and resistance of components.

02. Fault Analysis and Solutions for Common Instruments

1. Temperature Instruments (Thermocouples & Thermal Resistors)

  • Thermocouples: Errors may arise from mismatched compensation cables or incorrect polarity.
  • Thermal Resistors: Suitable for low-temperature environments. For measurement errors (e.g., max/min readings), first check the resistance or thermoelectric signal at the sensor. Then inspect for short circuits, open circuits, or poor contact in wiring.

2.Dual-Flange Differential Pressure Transmitters for Liquid Level Measurement

Measurement
Common issues post-installation include significant measurement errors or fluctuating readings. Causes:

  • Capillary tube rupture causing silicone oil leakage.
  • Silicone oil expansion/contraction due to ambient temperature changes.
  • Medium crystallization or high viscosity.
  • Process-related density variations.

Solutions:

  • Stabilize capillary temperature with insulation.
  • Recalibrate the transmitter zero point for sensor drift.
  • Adjust transmitter range parameters for density changes

3.Control Valves
Common issues:

  • Valve fails to actuate: Check air supply (blockages, pressure), DCS output, diaphragm leaks, or stem jamming.
  • Slow actuation: Inspect stem/valve core deformation, diaphragm rupture, or low air pressure.
  • Excessive leakage: Replace worn valve cores, adjust stem length, replace corroded springs, or clear blockages.
  • Fluctuations: Optimize PID parameters, secure linkage between positioner and stem, fix air leaks, or repair signal lines.

4.Flow Meters (No Display)
Check:

  • Signal/power line connections.
  • Loose display module pins.
  • Damaged transformers or fuses.
  • Grounding issues in signal lines.

5. Pressure Instruments

  • Fluctuating readings: Confirm process parameter changes or PID anomalies.
  • Static readings: Check for blocked impulse lines or faulty output modules.
  • Inaccurate readings: Relocate instruments away from electromagnetic interference.

6. Signal Cable Faults
Causes:

  • Cable damage during installation (e.g., abrasion, insulation degradation).
  • Proximity to high-temperature equipment leading to burnout.

Preventive Measures

  • Avoid rough handling during cable laying.
  • Route cables away from high-temperature zones. Use protective conduits or trays.

Maintenance Recommendations

  • Establish and enforce regular inspection protocols.
  • Conduct daily checks for leaks, cleanliness, and mechanical wear. Lubricate moving parts and address corrosion promptly.
  • In corrosive/dusty environments, maintain seals, transmitters, and connectors to ensure reliability.

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